1. Field of the Invention
This invention relates to an image forming apparatus, such as an electrophotographic apparatus, an electrostatic recording apparatus or the like, and more particularly, to an image forming apparatus which is suitable when a transfer material is adsorbed or adhered onto a carrying sheet for electrostatically transferring a toner image on an image carrying member to the transfer material held on the carrying sheet.
2. Description of the Related Art
In a color electrophotographic copier, a transfer drum is configured by winding a carrying member generally in the form of a carrying sheet such as a high-resistance film, around a drum-like frame. A transfer material is supplied to the transfer drum and wound therearound, and an end of the transfer material is mechanically fixed and held by a gripper disposed at a part of the circumferential surface of the transfer drum. Alternatively, the transfer material is fixed and held on the carrying sheet by electrostatically providing an adsorption force between the carrying sheet and the transfer material while winding the transfer sheet (as described in Japanese Patent Public Disclosure (Kokai) No. 55-32079 (1980)). In this way, an image supporting member (the transfer material) is carried by carrying means (the transfer drum) to a transfer position.
In the process of rotating the transfer drum a plurality of times, a toner image having plural colors is electrostatically transferred from an image carrying member (such as a photosensitive drum) to the surface of the transfer material. For this purpose, a corona charging device is disposed behind the carrying sheet at the transfer position, or a conductive roller is disposed in rotational contact with the transfer material held on the carrying sheet, whereby electric charges having a polarity opposite to that of the toner image are supplied to the transfer material. Subsequently, in order to discharge the transfer material from the carrying sheet, the electrostatic adsorption force (Coulomb force) produced between the transfer material and the carrying sheet is weakened using a corona discharger, and a separation claw is inserted between the transfer material and the carrying sheet to separate the transfer material.
In the conventional image forming apparatus as described above, however, a gap is occasionally produced locally between the transfer material and the carrying sheet when holding the transfer material on the carrying sheet. This happens when a transfer material holding surface of the carrying sheet is deformed. Such deformations are commonly caused by a cleaning member which bears against the carrying sheet when cleaning the carrying sheet, electrostatic forces produced by charging of the carrying sheet, thermal deformations from a fixing device situated near the moving region of the carrying sheet, the reaction of the carrying sheet when the transfer material contacts the carrying sheet, and the like. Transfer efficiency decreases where there is a gap, and there may arise a region where transfer is not achieved at all. Furthermore, when there is a large deformed region in the carrying sheet, jamming easily occurs, especially if the carrying sheet is convex relative to the transfer material holding surface. Finally, gaps decrease the effectiveness of the electrostatic adsorption of the transfer material, producing deviation in the position of the transfer material on the carrying sheet. These phonomena are even more pronounced when the transfer material is held on the carrying sheet with an electrostatic adsorption force, i.e. without using a mechanical gripper.
If portions of the transfer material become detached from the carrying sheet after image transfer, contamination of the image transferred occasionally occurs, caused by the contact of the transfer material with members around the transfer drum which disturbs the image on the transfer material.